Conventionally, there are known printing heads which discharge ink drops from a plurality of nozzles to a printing medium to form an image comprised of a plurality of printed ink dots. In order to form a high-definition, high-quality gradation image by a printing head of this kind, it is necessary to vary the printed ink dot diameter over a range of sizes by changing the size of the ink drops discharged from the printing head.
Interestingly, there exists a growing perception that nozzles of known printing heads have an upper limit and a lower limit with regard to the size of an ink drop which can be stably discharged therefrom. Consequently, there has been proposed a printing head having at least one small diameter nozzle, which discharges a small ink drop, for the formation of an ink dot in a small area and a large diameter nozzle, which discharges an ink drop larger than that of the small nozzle, for the formation of an ink dot in a large area. Examples of such a printing head are disclosed in U.S. Pat. No. 5,208,605 and U.S. Pat. No. 5,412,410.
Today's printers are commonly required to perform both gradation printing as well as text printing. Gradation printing is the printing of images which require gradational expression, while text printing consists of those images requiring no gradation expression, for example, common text and line drawings. While gradation printing utilizes particular dot arrangements and varied dot diameters to form images, text printing requires neither the level of detail required for gradation printing nor a variation in the size of printed dot diameters used to form the "text" images. Rather, high-speed text printing forms images using largely a single printed dot diameter.